Romania entered the war in an attempt to seize Transylvania, an area in Austria-Hungary with a Romanian ethnic majority but under Hungarian administration. Despite initial successes, the combined Russo-Romanian forces suffered several setbacks, and by the end of 1916 only Moldavia remained under Allied control. After several defensive victories in 1917, with different groups competing for authority over the Russian troops in Romania in the aftermath of the October Revolution, Romania signed an armistice at Focșani. On November 10, 1918, just one day before the German armistice and when all the other Central Powers had already capitulated, Romania re-entered the war. By then, about 220,000 Romanian soldiers had been killed, representing about 6% of total Entente military deaths.

In return for entering the war on Allied side, the Kingdom of Romania demanded support for its territorial claims over Transylvania, an Austro-Hungarian territory with a Romanian majority. The Romanians' greatest concerns in negotiations were to avoid being left to herself fighting on two fronts (one in Dobruja with Bulgaria and one in Transylvania), and to obtain written guarantees of Romania’s territorial gains after the war. To do this there were to be the following guarantees: a no-separate peace clause, equal status at the future peace conference, Russian military assistance against Bulgaria, an Allied offensive in the direction of Bulgaria, and the regular shipment of Allied war supplies. The military convention signed stipulated that France and Britain should start an offensive against Bulgaria and the Ottoman Empire no later than August, that Russia would send troops into Dobruja, and that the Romanian Army would not be subordinated to Russian command. The Allies were to send 300 tons of provisions on a daily basis. According to the Romanian account, most of these clauses, with the exception of those imposed on Romania, failed to be respected.[12]

The Allies accepted the terms late in the summer of 1916 (see Treaty of Bucharest, 1916); Cyril Falls attributes the late decision to Romania's historical hostility towards the Russian Empire, and purports an earlier entry into the war, such as before the Brusilov Offensive, would have provided better chance for victory.[13] According to some American military historians, Russia delayed approval of Romanian demands out of worries about Romanian territorial designs on Bessarabia, claimed by nationalist circles as a Romanian land.[14] According to British military historian John Keegan, before Romania entered the war the Allies had secretly agreed not to honour the territorial expansion of Romania when the war ended.[15]

Sketch pleading for the entry of Romania into war against Austria-Hungary in order to create Greater Romania (Ilustraţiunea magazine, November 1915)

In 1915 Lieutenant-Colonel Christopher Thomson, a fluent French speaker, was sent to Bucharest as British military attaché on Lord Kitchener's initiative to bring Romania into the war. Once there he quickly formed the view that an unprepared and ill-armed Romania facing a war on two fronts would be a liability, not an asset to the Allies. This view was brushed aside by Whitehall, and he signed a Military Convention with Romania on 13 August 1916. Within a few months he had to alleviate the consequences of Romania’s setbacks, and supervised the destruction of the Romanian oil wells to deny them to Germany (later Thomson was a Labour peer and Secretary of State for Air).[16]

The Romanian government signed a treaty with the Allies (France, Britain, Italy and Russia) on 17 August 1916, pledging to declare war on Austria-Hungary by 28 August. The Romanian ambassador in Vienna actually transmitted the declaration of war on 27 August. Germany, caught by surprise, responded with a declaration of war on Romania the next day (28 August). The dates of the Bulgarian and Ottoman declarations of war are disputed. Ian Beckett says that Bulgaria did not issue a declaration of war prior to its attack of 31 August.[17] Other sources place the declaration on 30 August[18] or 1 September.[19][20] The Ottoman declaration took place either on 29 August,[18] 30 August[19] or 1 September.[20] Within two days of her own declaration, according to one source, Romania found herself at war with all the Central Powers.[21]

The Romanian Army was quite large, with over 650,000 men in 23 divisions, but it suffered from poor training and equipment, particularly when compared to its German counterparts. Meanwhile, the German Chief of Staff, General Erich von Falkenhayn, had correctly reasoned that Romania would side with the Allies, and had made plans accordingly. Thanks to the earlier conquest of the Kingdom of Serbia and the ineffective Allied operations on the Kingdom of Greece border, and having a territorial interest in Dobruja, the Bulgarian Army and the Ottoman Army were willing to help fight the Romanians.

The German high command was seriously worried about the prospect of Romania entering the war, Hindenburg writing:

It is certain that so relatively small a state as Rumania had never before been given a role so important, and, indeed, so decisive for the history of the world at so favorable a moment. Never before had two great Powers like Germany and Austria found themselves so much at the mercy of the military resources of a country which had scarcely one twentieth of the population of the two great states. Judging by the military situation, it was to be expected that Rumania had only to advance where she wished to decide the world war in favor of those Powers which had been hurling themselves at us in vain for years. Thus everything seemed to depend on whether Rumania was ready to make any sort of use of her momentary advantage.[22]

The war which for the last two years has been encircling our frontiers more and more closely has shaken the ancient foundations of Europe to their depths.

It has brought the day which has been awaited for centuries by the national conscience, by the founders of the Romanian State, by those who united the principalities in the war of independence, by those responsible for the national renaissance.

It is the day of the union of all branches of our nation. Today we are able to complete the task of our forefathers and to establish forever that which Michael the Great was only able to establish for a moment, namely, a Romanian union on both slopes of the Carpathians.

For us the mountains and plains of Bukowina, where Stephen the Great has slept for centuries. In our moral energy and our valour lie the means of giving him back his birthright of a great and free Rumania from the Tisza to the Black Sea, and to prosper in peace in accordance with our customs and our hopes and dreams.

Romanians!

Animated by the holy duty imposed upon us, and determined to bear manfully all the sacrifices inseparable from an arduous war, we will march into battle with the irresistible élan of a people firmly confident in its destiny. The glorious fruits of victory shall be our reward. Forward, with the help of God!

On the night of August 27, 1916, three Romanian armies (First, Second and Northern), deployed according to the Romanian Campaign Plan (The "Z" Hypothesis), launched attacks through the Carpathians and into Transylvania.[24] Initially the only opposing force was the Austro-Hungarian First Army, which was steadily pushed back toward Hungary. In a relatively short time, the towns of Brașov, Făgăraș and Miercurea Ciuc were captured and the outskirts of Sibiu were reached. In Romanian populated areas, the Romanian troops were warmly welcomed by the population, which provided them considerable assistance in terms of provisions, billeting or guiding.[25] However, the rapid Romanian advance alarmed the Central Powers, and within weeks sizable reinforcements began arriving at the scene. The Entente incorrectly assumed that Germany would be unable to respond to the invasion, as the Battle of the Somme and the Brusilov Offensive were at their height around this time and tied down significant German forces. Nevertheless, eight divisions and an Alpine Corps were deployed under the command of Erich von Falkenhayn. The Austro-Hungarians also sent four divisions to reinforce their lines and by the middle of September, the Romanian offensive was halted. The Russians loaned them three divisions for operations in the north of Romania but hampered their efforts by failing to provide much-needed supplies.

Roter-Turm-Pass
"Deutsches Alpenkorps
26.-29.9.1916"

While the Romanian Army was advancing in Transylvania, the first counterattack came from Field MarshalAugust von Mackensen in command of a multi-national force composed of the BulgarianThird Army, a German brigade and two divisions of the Ottoman VI Army Corps, whose units began arriving on the Dobrudja front after the initial battles.[26] This army attacked north from Bulgaria, starting on September 1. It stayed on the south side of the Danube river and headed towards Constanța. The Romanian garrison of Turtucaia, encircled by Bulgarian troops (aided by a column of German troops) surrendered on September 6. The Romanian Third Army made further attempts to withstand the enemy offensive at Silistra, Dobrich, Amzacea and Topraisar, but had to withdraw under the pressure of superior enemy forces. Mackensen's success was favoured by the Allies' failure to fulfill the obligation they had assumed through the military convention, by virtue of which they had to mount an offensive on the Macedonian front and the conditions in which the Russians deployed insufficient troops on the battlefront in the south-east of Romania. These factors meant that the Romanian forces became too strained to put up effective resistance against the enemy advance. Romania had to fight on two 1,600 km-long battlefronts, the longest front in Europe, with a varied configuration and diverse geographical elements (by comparison, the Russian front, stretching from the Baltic Sea until Bukovina, was only 1,000 km long).[27]

Romanian troops in Transylvania, 1916

On September 15, the Romanian War Council decided to suspend the Transylvania offensive and concentrate on the Mackensen army group instead. The plan (the so-called Flămânda Maneuver) was to attack the Central Powers forces from the rear by crossing the Danube at Flămânda, while the front-line Romanian and Russian forces were supposed to launch an offensive southwards towards Cobadin and Kurtbunar. Russian reinforcements under General Andrei Zaionchkovsky arrived to halt Mackensen's army before it cut the rail line that linked Constanța with Bucharest. Fighting was furious with attacks and counterattacks up till September 23. The Bulgarian Third Army suffered a tactical defeat in the first battle of Cobadin on September 19, forcing the Central Powers to temporarily halt their advance until mid-October. On October 1, two Romanian divisions crossed the Danube at Flămânda and created a bridgehead 14 kilometer-wide and 4 kilometer-deep. On the same day, the joint Romanian and Russian divisions went on offensive on the Dobruja front but with little success. The failure to break the Dobruja front, heavy fighting in the Flămânda area on October 3, and a heavy storm on the night of October 1 which damaged the pontoon bridge, determined General Alexandru Averescu to cancel the whole operation. This would have serious consequences for the rest of the campaign.

Overall command was now under Falkenhayn (recently replaced as German Chief of Staff), who started his own counterattack on September 18. The first attack was on the Romanian First Army near the town of Hațeg; the attack halted the Romanian advance. Eight days later, German troops attacked Sibiu, and on September 29 the outnumbered Romanians began retreating to the Vulcan and Turnu Roșu passes; the latter, however, had been occupied by Bavarian mountain troops in a flanking movement, and a violent battle followed that ended with the Romanians retaking the pass at a cost of 3,000 men. On October 4, the Romanian Second Army attacked the Austro-Hungarians at Brașov but the attack was repulsed and the counterattack forced the Romanians to retreat from there also. The Romanian Fourth Army, in the north of the country, retreated without much pressure from the Austro-Hungarian troops so that by October 25, the Romanian Army was back to its initial positions.[28] Concentrating important military forces rapidly brought over from the other theatres of operations in Europe, and as a result of the fact that a great number of Romanian units had to be quickly shifted to the battlefront in Dobruja, the enemy succeeded in taking the strategic initiative in Transylvania.[27]

In October 1916 the Romanian army mounted a wide-scale operation, the main target of which was the defense of the mountain passes in the Southern and Eastern Carpathians against the ever stronger pressure of the German-Austro-Hungarian forces. Grim fights were delivered in the Prahova Valley, where occupation of the locality of Predeal was one of the major aims pursued by the Central Powers. Given their dramatic character, the clashes for the Predeal town and railway station were many a time compared with the heaviest fights on the Western Front. Similar fights took place in the Bran-Câmpulung area, especially at Dragoslavele and Racoș.[27]

Particular heed was paid to the actions carried on for the defense of the Carpathians' alignment, the fights on the Jiu River. There, the Germans had massed large forces, pursuing to beat their way south of the mountains. Faced with the enemy threat, the troops of the Romanian First Army, under command of General Ion Dragalina, offered strong resistance. Everywhere the Romanian soldiers were supported by the civil population as, for instance, during the Battle of Târgu Jiu, when the town was defended by its inhabitants, men, women and children, young and old. There, a conspicuous figure was cut by Ecaterina Teodoroiu, who was to enter the consciousness of all Romanians as the "Heroine of the Jiu". The operation for the defense of the Carpathians holds a prominent place in Romanian military history not only because it was one of the most difficult operations waged by the Romanian army until then, but also because it was one of the most important as regards the complexity of the actions carried on and the highly valuable lessons derived from their evolution.[27]

After the Romanian troops were initially able to stop the German advance on the Jiu Valley, on the 16 October 1916 the German army regrouped. The German High Command created the Army Group Kühne headquartered in Petroșani, under the command of General Viktor Kühne. This Army Group included the 11th and 301st Bavarian infantry divisions, which had previously fought the Romanians on the Jiu, the 41st Prussian and the 109th infantry divisions which were transferred from the Riga front as well as the newly formed 58th Cavalry Corps (z.b.V) under the command of general Egon von Schmettow, which included the 6th and 7th cavalry divisions. The German reserves consisted of the 115th infantry division and two brigades of cyclists. The total manpower of the Army Group amounted to 80,000 troops with 30,000 horses. The Romanian forces could not withstand the new German attack which started on 29 October 1916. The Romanians retreated and on 8 November 1916 the German cavalry entered Craiova. The Romanian army continued its retreat towards the Olt River, having the cavalry trying to slow the German advance, to give it time to organize a defensive line along the Olt. While the Romanian army made attempts to stop the advance of the German forces, such as the Battle of Robănești, these were largely unsuccessful. [29]

Back on the coast, Field MarshalMackensen and Bulgarian General Stefan Toshev launched a new offensive on October 19, after a month of careful preparations, and achieved a decisive victory in the Second Battle of Cobadin. The Romanians and Russians were forced to withdraw out of Constanța (occupied by the Central Powers on October 22). After the fall of Cernavodă, the defense of the unoccupied Dobruja was left only to the Russians, who were gradually pushed back towards the marshy Danube Delta. The Russian Army was now both demoralized and nearly out of supplies. Mackensen felt free to secretly pull a large number of troops back to the town of Svishtov in Bulgaria with an eye towards crossing the Danube river.

Falkenhayn's forces made several probing attacks into the mountain passes held by the Romanian Army to see if there were weaknesses in the Romanian defences. After several weeks, he concentrated his best troops (the elite Alpen Korps) in the south for an attack on the Vulcan Pass. The attack was launched on November 10. One of the young officers was the future Field Marshal Erwin Rommel. On November 11, then-Lieutenant Rommel led the Württemberg Mountain Company in the capture of Mount Lescului. The offensive pushed the Romanian defenders back through the mountains and into the plains by November 26. There was already snow covering the mountains and soon operations would have to halt for the winter. Advances by other parts of Falkenhayn's Ninth Army also pushed through the mountains; the Romanian Army was being ground down by the constant battle and their supply situation was becoming critical.

On November 23, Mackensen's best troops crossed the Danube at two locations near Svishtov. This attack caught the Romanians by surprise and Mackensen's army was able to advance rapidly towards Bucharest against very weak resistance. Mackensen's attack threatened to cut off half the Romanian army and so the Romanian Command prepared a counter-offensive known under the name of the Battle of Argeş-Neajlov (part of the Battle of Bucharest) and designated the recently promoted General Constantin Prezan to lead it. The plan envisaged the checking of the advance of the German Ninth Army from the north and north-west, as well as the encirclement and annihilation of the German-Bulgarian-Turkish units deployed south-east of Bucharest.[30] It was a bold undertaking, using the entire reserves of the Romanian Army, but it needed the cooperation of Russian divisions to contain Mackensen's offensive while the Romanian reserve struck the gap between Mackensen and Falkenhayn. However, the Russian Army disagreed with the plan and did not support the attack.

On December 1, the Romanian Army went ahead with the offensive along the Argeș and Neajlov rivers. Initially, the Romanians experienced success, taking a large number of prisoners, however Mackensen was able to shift forces to deal with the sudden assault and Falkenhayn's forces responded with attacks at every point.[31] Faced with the overwhelming superiority of the invading forces, the Romanian army, its ranks thinned from the previous actions, inferior in equipment and lacking Russian support, failed to check the enemy advance. Although it recorded numerous daring actions (among these the Prunaru Charge, in which the 2nd Roşiori Cavalry Regiment was almost wiped out), the Battle of Argeş-Neajlov ended unfavourably for the Romanian army.[30] Within three days, the attack had been shattered and the Romanians were retreating everywhere. Bucharest was captured on December 6 by Falkenhayn's cavalry. The Romanian Second Army made a fighting retreat to the Siret river, which had originally been fortified against the Russians and was facing the wrong direction but nevertheless would end up proving invaluable, protected by the impassable Danube Delta to the southeast and a flank in the Carpathians in the northwest. Fierce fighting took place at Râmnicu Sărat between December 22–26, with Mackensen's forces entering the town on December 27. Around this time, the Russians began sending numerous reinforcements to Moldavia to prevent an invasion of southern Russia. Southern Romania, including Oltenia, Muntenia, Dobruja and southern Moldavia, was now in the hands of the Central Powers. While retreating, the Romanians burnt stores of grain and destroyed oil wells to prevent them from being used by the Germans.[30]

The remaining Russo-Romanian forces in Dobruja abandoned Măcin on January 4 and Brăila on January 5. Toward the end of the month, extreme frost gave the Bulgarians an opportunity to enter the Danube Delta. On January 23, they attempted to cross the marshes at Tulcea, but suffered heavy casualties to Romanian defenders on the northern bank and stopped. Fighting also ceased in the Carpathian passes, also owing to unfavorable weather. Mackensen's troops were able to capture Focșani on January 8, but an attempt to break the Siret River line on January 19 failed. Thus, the front stabilized and allowed for the Romanian army to be refitted and rebuilt.

Romania entered the war at a time of strong crisis for the Entente, drawing upon itself numerous enemy forces, fighting on a battlefront the total length of which placed it before the other fronts in Europe and having to change permanently its initial campaign plan. However, in spite of the human, material and military efforts made by the Central Powers throughout this period, they failed to achieve their fundamental political and strategic goal, namely Romania's defeat and her getting out of the war. Despite heavy casualties, some 250,000 men, which were almost one third of the manpower mobilized in August 1916, and losses of combat material, the Romanian army was still a force taken into consideration by allies and enemies alike and capable to offer resistance to further attacks. Part of the population moved to the free territory, together with the Romanian government, royal court and public authorities which relocated to Iași. Therefore, the Kingdom of Romania continued to exercise the attributes of an independent and sovereign state, allied to the Entente powers.[30]

In 1917, when both belligerent sides were making huge efforts to win the final victory, for Romania it was vitally important to defeat the occupants, as the liberation of the occupied territory, the existence of the Romanian state and the completion of its unity depended on it. After the Romanian troops, in cooperation with Russian military forces, had managed to bring the enemy to a halt at Moldavia's Gates, on the Eastern Carpathians, the Siret River and the Danube Delta alignment, Romania embarked during the first half of 1917 upon the reconstruction and strengthening of its combat capability through multiple national efforts under highly complex international circumstances. Considerable measures were taken in all economic branches to rebuild the evacuated factories and workshops, to increase the production destined for the national defense and the productivity yielded by the exploitation of the few petroleum and coal resources in the free zones. The agriculture received special attention, as it needed to meet the food needs and ensure a minimal living standard to the population in the free part of the country, to the refugees who had left their houses in front of the enemy invasion and also to the Romanian army and the Russian troops (about one million by early 1917).[32]

With a view to achieving the unity of action of the internal political forces that was indispensable to safeguarding the nation's interests, a government of national union was set up in Iaşi on December 24, 1916, led by Ion I. C. Brătianu. The political life in unoccupied territory had, therefore, as its fundamental aim to achieve national consensus with a view to initiating and materializing the best steps to take in various sectors of public life to make possible the successful continuation of the liberation war. Within this framework, the debates on and endorsement of some laws envisaging structural transformations (primarily the agrarian reform, through which land was allotted to the peasants and the election law, which introduced universal suffrage) answered the popular demands, contributing thereby to enhancing the morale of the soldiers in the front lines.[32]

The Romanian army's reconstruction was to involve both its reorganization and modernization. While the forces that had taken part in the big battle of Bucharest (Army Group Prezan) were reshuffled inland Moldavia, the Romanian Second Army, which had preserved to a great extent its combat structures and force, remained on the front in southern Moldavia, where, alongside Russian forces, it checked the enemy advance. The reorganization was initiated by King Ferdinand and the Romanian government and was carried on under their leadership and control in the free national territory, in spite of Russian attempts to shift the Romanian army beyond the Dniester, inside Ukraine. This complex and long process was underlain by a set of orders and measures that were the result of Romanian combat experience and also by the conclusions offered by more than two years of fighting on the other fronts in Europe. The reorganization pursued the reduction of the effectives of the "Operations Army" to such parameters that suited the country's resources for waging a long campaign. The infantrydivisions were ensured identical structure to make replacements and maneuvers easier on the battlefront and to have a fire power comparable with that of the enemy. The army corps became only a command body for tactical coordination. The cavalry divisions received more machine guns. The artillery material underwent a homogenization process, with two regiments (one cannon, the other howitzer) for each division, while the heavy artillery was organized as a distinct group.[33]

The reorganization also involved the other troops (combat engineers, air force, navy) and services which underwent notable improvements. The directions, organization and methodology of the training of the command staff and the troops were considerably improved and special training centers were set up. Priority was given to engineer works in the field, to the assimilation of new technology and to night combat, with repeated drills up to division level when combat ammunition was used so as to have the soldiers accustomed to real warfare conditions as much as possible.[34]

Aware of the complex strategic situation, the Romanian Command lent its military policy a clear, realistic orientation, that is of committing the entire population to battle, trying to act efficiently in keeping with the national goals and in harmony with the large-scale operations worked out at the coalition level. Its final form ready in late May 1917, the operations plan for the Romanian front contemplated the mounting of a general offensive in the Focşani-Nămoloasa sector with a view to completely pin down all enemy forces there, to annihilate the main enemy groups operating there (the German Ninth Army) and to support the Kerensky Offensive.[36] The decisive effort was to be made by the Romanian First Army. In order to increase the effect of the offensive and to draw as many enemy troops as possible north-west of the town of Focşani, the actions of the Romanian Second and Russian Fourth Armies had to precede those of the Romanian First Army. The Oberste Heeresleitung, which had moved the center of gravity of its military operations to the Eastern Front, aiming to win the victory there through the defeat of Romania and the conclusion of a peace with Russia, decided in June 1917 to mount a wide-scope offensive in the north and south of Moldavia, to which end it brought over reinforcements from the other fronts.[37]

In early July 1917, on the Romanian front, a relatively small area, there was one of the largest concentrations of combat forces and means known during the conflagration: nine armies, 80 infantry divisions with 974 battalions, 19 cavalry divisions with 550 squadrons and 923 artillery batteries, whose effectives amounted to some 800,000 men, with about one million in their immediate reserve. The three great battles, decisive for the Romanian nation's destiny, delivered at Mărăști, Mărășești and Oituz represented a turning point in the world war on the Eastern front. These battles, named by the localities and zones where they took place, were fought approximately on the front alignment stabilized in early 1917, which the conflicting sides had thoroughly consolidated for half a year.[38]

The offensive of Mărăști started, after two days of strong artillery preparations, on July 24, 1917, at dawn, and took place in the Vrancea County, in the sector of the Romanian Second and Russian Fourth Armies. Assumed with three divisions, by surprise, the offensive succeeded in disrupting the well-organized enemy defenses, compelling the Austro-Hungarians and Germans to retreat. By the evening, the Romanian divisions had conquered the first defenses, the strongest and deepest of the defensive system of the Gerok Group (belonging to the Austro-Hungarian First Army) in the Mărăști area. The next day, pursuing the offensive, the Romanian troops forced the enemy into an ever more disorderly retreat, favorable conditions being thus created for an in-depth penetration into the defensive disposition and the annihilation of the enemy group. However, under the circumstances in which the Stavka decided unilaterally to stall any offensive as a result of the grave situation created on the front in Galicia and Bukovina following the failure of the Kerensky Offensive and the counter-attack of the Central Powers' forces, the Romanian General Headquarters saw itself compelled to discontinue the offensive throughout the entire territory between the Eastern Carpathians and the Black Sea. In the Mărăști zone however, the Romanian units continued the offensive until July 30 upon the request of their commander, General Alexandru Averescu. This marked the end of the Battle of Mărăști. It inflicted important losses upon the Austro-Hungarians and Germans, who relinquished a 35 km-wide and 20 km-deep area, sustaining heavy casualties and losses in combat means. The offensive potential of the Romanian army was confirmed through this victory.[38]

The salient created by the Romanian troops in the enemy lines at the junction between the Austro-Hungarian First Army and the German Ninth Army made the High Command of the Central Powers bring forces from other sectors on the Moldavian front and change the main direction of the offensive initially planned for the Focşani-Nămoloasa region. After the Mărăști operation had been discontinued, the Central Powers tried to implement their offensive plan in the summer of 1917. They pursued to encircle and smash the Romanian and Russian forces through a blow dealt north-westward, in the direction of Focşani, Mărășești and Adjud (an operation which led to the Battle of Mărășești), conjugated with another blow that had to start from the mountains, through the Oituz and Trotuș valleys, towards Târgu Ocna and Adjud (the Second Battle of Oituz). Pursuing the offensive, the German troops aimed at occupying the whole of Moldavia, thereby knocking Romania out of the war, and, together with an in-depth penetration of the Austro-Hungarian troops on the front in Bukovina, to push the Russian forces eastwards, beyond Odessa. The offensive of the German Ninth Army, from the Army GroupMackensen, started on August 6, 1917, when the units of the Russian Fourth Army on the Siret River were expected to leave their positions to reinforce the front in the north of Moldavia and be replaced by the divisions of the Romanian First Army (commanded by General Constantin Cristescu until August 12, then by General Eremia Grigorescu).[39]

For 29 days, until September 3, this sector was the scene of the most important battle delivered by the Romanian army during the 1917 campaign. The Battle of Mărășești had three distinct stages. During the first stage (August 6-12), successively committed to battle, the troops of the Romanian First Army, together with Russian forces, managed to arrest the enemy advance and forced the Germans, through their resistance, to gradually change the direction of their attack north-westward. In the second stage (August 13-19), the Romanian Command completely took over the command of the battle from the Russians and the confrontation reached its climax on August 19, ending in a complete thwarting of the enemy's attempts to advance. The third stage (August 20 - September 3) actually saw the last German attempt at least to improve their positions in view of a new offensive, this one too baffled by the Romanian response.[40]

Starting with August 8, 1917, the fighting on the Mărășești front combined with an Austro-Hungarian-German offensive at Oituz. Holding out against superior enemy forces, by August 30 the Romanian troops stemmed the advance of the Gerok Group, successively reinforced with numerous forces and means, which only managed to achieve 2-6 km-deep and 18-20 km-wide breakthrough in the defensive disposition of the Romanian Second Army. The definitive discontinuing of the Central Powers' general offensive on the Romanian front on September 3, 1917 therefore marked their strategic defeat and a considerable weakening of their forces on the South-Eastern front, the response given by the Romanian army being actually the strongest blow they were dealt in East Europe in 1917.[40]

As a result of these operations, the remaining Romanian territories remained unoccupied, tying down nearly 1,000,000 Central Powers troops and prompting The Times to describe the Romanian front as "The only point of light in the East".

The situation, however, once again took a turn for the worse for the Entente in November with the October Revolution and the beginning of the Russian Civil War. These events effectively ended Russian involvement in the war, and Romania was left isolated and surrounded by the Central Powers and had little choice but to negotiate the Focsani Armistice, signed by the combatants on December 9, 1917.

On May 7, 1918, in light of the existing politico-military situation, Romania was forced to conclude the Treaty of Bucharest with the Central Powers, imposing harsh conditions on the country but recognizing its union with Bessarabia. Alexandru Marghiloman became the new German-sponsored Prime Minister. King Ferdinand, however, refused to sign the treaty.

The Germans were able to repair the oil fields around Ploiești and by the end of the war had pumped a million tons of oil. They also requisitioned two million tons of grain from Romanian farmers. These materials were vital in keeping Germany in the war to the end of 1918.[41]

After the successful offensive on the Thessaloniki front which put Bulgaria out of the war, Romania re-entered the war on November 10, 1918, a day before its end in Western Europe.

On November 28, 1918, the Romanian representatives of Bukovina voted for union with the Kingdom of Romania, followed by the proclamation of the union of Transylvania with the Kingdom of Romania on December 1, 1918, by the representatives of Transylvanian Romanians gathered at Alba Iulia, while the representatives of the Transylvanian Saxons approved the act on December 15 at an assembly in Mediaș. A similar gathering was held by the Hungarians in Cluj, on the 22 of December. They reaffirmed their allegiance to Hungary.

The Treaty of Versailles recognized these proclamations under the right of national self-determination (see the WilsonianFourteen Points). Also Germany agreed under the terms of the same treaty (Article 259) to renounce to all the benefits provided by the Treaty of Bucharest in 1918.[42]

Percentage of military deaths during World War I for the Entente Powers

The 1916 counteroffensive was an impressive feat for the German Army and their generals Falkenhayn and Mackensen.[44] Despite this the Germans represented only 22% of the Central Power's forces that took part in the campaign compared to the Austro-Hungarian 46% and combined Bulgarian and Ottoman 32%.[45]

General Vincent Esposito argues the Romanian high command made grave strategic and operational mistakes:

Militarily, Romania's strategy could not have been worse. In choosing Transylvania as the initial objective, the Romanian Army ignored the Bulgarian Army to her rear. When the advance through the mountains failed, the high command refused to economize forces on that front to allow the creation of a mobile reserve with which Falkenhayn's later thrusts could be countered. Nowhere did the Romanians properly mass their forces to achieve concentration of combat power.[44]

The failure of the Romanian front for the Entente was also the result of several factors beyond Romania's control. The failed Salonika Offensive did not meet the expectation of Romania's "guaranteed security" from Bulgaria.[46] This proved to be a critical strain on Romania's ability to wage a successful offensive in Transylvania, as it needed to divert troops south to the defense of Dobruja.[47] Furthermore, Russian reinforcements in Romania did not materialize to the number of 200,000 soldiers initially demanded.[48] Romania was thus placed in a difficult situation several months after it joined the war, with the Entente unable to provide the support it had promised earlier. All of these amounted to a costly stalemate, instead of the "decisive deathblow" to the Austro-Hungarian Empire that the Entente had expected.[citation needed]

^ abZlata Filipovic and Melanie Challenger, Stolen Voices: Young People's War Diaries, From World War I to Iraq (Doubleday Canada, 2006), 29, quoting German soldier Piete Kuhr's diary entry for 1 September 1916.